1. Field of the Invention
This invention relates to a magnetic recording medium and more particularly, it is concerned with a magnetic recording medium in which the surface smoothness of the coating, the degree of orientation in a magnetic field and the reproducing output are improved by improving the dispersion of a coating composition, shortening the dispering time and raising the stability of a coating composition during passage of time.
2. Description of the Prior Art
Up to the present time, magnetic recording media have widely been used for magnetic recording and reproducing, which are obtained by dispersing a ferromagnetic substance consisting of acicular crystals of .gamma.-Fe.sub.2 O.sub.3 or CrO.sub.2 in a binder and coating the resulting coating composition onto a non-magnetic support. Lately, however, improvement of the recording density has eagerly been desired so as to achieve the enlargement of a recording capacity and the miniaturization, but it is necessary to make the maximum size of an acicular magnetic grain sufficiently smaller than a recording wavelength or recording bit length in order to obtain a recording medium suitable for high density recording using an acicular magnetic powder as in the prior art. At the present time, an acicular magnetic powder with a size of about 0.3 .mu.m has been put to practical use to obtain a shortest recording wavelength of about 1 .mu.m.
For the purpose of obtaining a medium whereby a further high density recording is made possible in future, it will be required to make further small the size of an acicular magnetic powder. In such a small acicular magnetic powder, however, the thickness is extremely fine, e.g. 100 .ANG. or less and the grain volume is very small, e.g. 10.sup.-17 cm.sup.3 or less, which result in the problem that the magnetic properties are deteriorated by thermal agitation or surface effects and a sufficient orientation cannot be obtained even if a magnetic field is applied to the magnetic coating film.
In a magnetic recording medium coated with a coating composition in which an acicular magnetic substance having an acicular ratio (long axis/short axis) of more than 10 is dispersed, the direction of coating due to the acicular shape remains so that in the case of using the magnetic recording medium by rotating in the form of a disk, an output fluctuation occurs with its rotation period.
Accordingly, there have hitherto been proposed magnetic recording media each using, as a ferromagnetic substance, hexagonal ferrite which is plate-shaped and has the axis of easy magnetization in the vertical direction to the plate surface, and each being subjected to magnetic orientation in the longitudinal direction in the plane of the recording medium (Japanese Patent Application OPI (Kokai) Nos. 6525/1983 and 6526/1983). However, it is difficult to increase the dispersion of a ferromagnetic substance of plate-shaped hexagonal crystal system and to shorten the dispersion time because of its strong adsorption force, and moreover, it is difficult to obtain a magnetic recording medium with an excellent surface smoothness, high degree of orientation and high reproducing output because a coating composition containing the ferromagnetic substance, after dispersed, is inferior in stability during the passage of time, resulting in immediate reaggregation.